Tokyo Bay, Tokyo

Tokyo Bay is worth a day trip. The views are lovely, the trails are nice, and there are some impressive buildings, a few nice statues, and a couple museums.
Palette town will keep you busy for hours. It includes 3 very expensive malls, the cleanest MacDonalds I have ever been in, and the Tokyo Megaweb Exhibition.
If you have any interest in cars, you should check it out. Admission is free, and the few activities with a price tag are pretty cheap.
Visit the History Garage Museum to see famous and quirky old vehicles, view the concept car display, learn about hybrid engines and handicap accessability options, and take a ride in a car that drives itself! You'll exhaust yourself before you see everything.
Before it gets dark, make sure you take a ride on one of the world's largest ferris wheels. If you're lucky and have the stomach for it, you might even get a glass-bottomed compartment.